This invention relates to a method of concentrating tomato juice by reverse osmosis.
The method based on reverse osmosis is coming to be considered for concentrating tomato juice because concentrated tomato juice of a higher quality can be obtained by this method than by a more commonly practiced method by heating and evaporation. At the site of such a concentration work, it goes without saying that it is desirable to be able to produce highly concentrated tomato juice reliably and stably without overloading the apparatus for concentration by reverse osmosis, at a low production cost and under a simple process management. The object of this invention is to respond to such requirements.
Conventionally known methods of concentrating tomato juice by reverse osmosis include the single-stage single-pass methods, the multi-stage single-pass methods and the multi-stage circulation methods. By the multi-stage methods, units for concentration each having a plurality of tubular membrane modules are aligned in series in two or more stages and tomato juice is caused either to flow through such units in a straight flow or to circulate around therethrough. Fairly highly concentrated products are obtainable by such a multi-stage method but the disadvantage of this method is that the apparatus for the concentration is expensive and this adversely affects the cost of production. Moreover, while the process management is cumbersome, bacterial contamination at connections of the units and quality degradation are difficult to avoid. Thus, these methods have been considered impractical and the single-stage single-pass methods are more commonly practiced by which a concentration unit with a plurality of serially connected tubular membrane modules is provided only in one stage and tomato juice is caused to flow therethrough only once.
On the subject of concentrating tomato juice by reverse osmosis in a single-stage single-pass process, Japanese Patent Publication Tokko 61-48904 disclosed a method for controlling tomato juice in a laminar flow area with Reynold's number less than 3000, its linear speed at 80 cm/second or less at the entrance to the concentration unit and the pressure loss at less than 35 kg/cm.sup.2. Japanese Patent Publication Tokko 59-53824 disclosed another method of controlling the sludge volume of tomato juice at 5% or more and the pressure loss at less than 40 kg/cm.sup.2. By either of these methods, however, the apparatus for concentration by reverse osmosis tends to become overloaded occasionally and the problem remains that highly concentrated products cannot be obtained reliably and stably.